1. Field of the Invention
This invention relates to a method of protecting electrodialysis (ED) membranes and stack components, and, more specifically, this invention relates to a method for improving the productivity of ED cells and stacks and protecting ED membranes from the effects of acidic or basic solutions.
2. Background of the Invention
Electrodialysis was commercially introduced in the early 1960's. The development of electrodialysis provided a cost-effective way to desalt brackish water and spurred considerable interest in this area. Electrodialysis depends on the following general principles: 1) most salts dissolved in water dissociate into ions which are positively (cationic) or negatively (anionic) charged; 2) these ions are attracted to electrodes with an opposite electric charge; 3) membranes can be constructed to permit selective passage of either anions or cations.
An exemplary electrodialysis stack consists of alternating anion- and cation-exchange membranes separated by spacers or gaskets. The spacers consist of a sealing frame and a net in the active area, which is filled with an electrolyte. The spacer net prevents the membranes from touching each other. The stacked spacers align to construct two different principal channel systems, feed (diluate) and brine (concentrate). The spacers direct the feed and brine solutions into the corresponding chambers and promote flow distribution. There are several circuits in a stack due to channels formed by the combination of active sites in membranes and the channels in the spacers.
A set of two membranes and two spacers forms a cell or cell pair, and hundreds of cells can be installed in one stack. A stack has its cell membranes oriented so that the planes formed by the membranes are perpendicular to the resting surface of the wall. As such, the stack extends laterally, not vertically. The driving force for a stack is a direct current between anodes and cathodes situated at the two ends of the stack. In conjunction with the current, the membranes comprise a means for controlling the ingress and egress of ionic moieties to and from different compartments. The stack feed or diluate system and the concentrate or brine system are connected to external tanks which provide various electrolyte solutions continuously flowing through the ED stacks. During electrolysis, the diluate solution becomes depleted in salt, while the concentrate solution becomes more concentrated. As a result, this type of electrodialysis has also been referred to as concentration electrodialysis. Similar two compartment ED cells are shown in FIGS. 1, 3-9, and 11 in U.S. Pat. No. 6,461,491 awarded to Hryn, et al. on Oct. 8, 2002 and incorporated herein by reference.
Another type of electrodialysis utilizes bipolar membranes. Electrodialysis with bipolar membranes, also referred to as water splitting membranes, is an efficient way to produce acids and bases from salt solutions. Under the influence of an electric field, bipolar membranes can split water into hydrogen ions (H+) and hydroxyl ions (OH−). A typical three compartment ED cell is depicted as numeral 8 in FIG. 1. The cell comprises alternating cation-selective membranes 12, bipolar membranes 20, and anion-selective membranes 14 under an electrical potential. A salt solution (M+X−) 9 flowing in the salt compartment defined by the anion-selective and cation-selective membranes, generates an acidic solution (H+X−) in an acid compartment between the bipolar and anion-selective membranes. Concomitantly a basic solution (M+OH−) forms in a base compartment defined by the bipolar and cation-selective membranes, as shown in FIG. 1.
Two-compartment ED cells that utilize bipolar membranes are also common. These arrangements are typically used to split salts of weak acids and strong bases, such as sodium acetate, or split salts of strong acids and weak bases, such as ammonium chloride. For the weak-acid/strong-base case (e.g., sodium acetate), alternating bipolar and cation-selective membranes are used, while for the strong-acid/weak-base case (e.g., ammonium chloride), alternating bipolar and anion-selective membranes are used. The configuration for the strong-acid/weak-base case is shown in FIG. 2. The product base (e.g. ammonium hydroxide) is mixed with the feed salt 9 in the base compartment (e.g. ammonium chloride), while the product acid (e.g. hydrochloric acid) forms in the acid compartment. These solutions recirculate through the ED stack causing the acid loop solution (solution flowing through the acid compartment) to become more concentrated in acid, while the base loop solution becomes more basic and depleted in the salt.
Chemical synthesis can be carried out via ED in a two-compartment ED cell using bipolar membranes and anion-selective membranes. An exemplary cell is depicted in FIG. 3 as numeral 10 for the case where anion-selective membranes are used. Bipolar membranes 20 comprise an anion-permeable membrane 14 and a cation-permeable membrane 12 laminated together. This laminated construct is similar to the bipolar membrane of FIGS. 1 and 2 whereby a water channel exists. When this composite structure is oriented to have a surface of the cation-exchange layer face the anode (positive pole) 15, the imposition of an electrical potential from the anode 15 across the membrane to the cathode 17, splits water into protic (H+) and hydroxyl (OH−) ions. This configuration produces acidic 19 and basic 21 solutions at the surfaces of the bipolar membranes. Accordingly, the pH of a solution passing through the components of the stack of such an ED system becomes more acidic or more basic.
The extent to which pH actually decreases or increases is determined by the stack current density, i.e., the rate at which water is split and H+ and OH− ions are generated, the electrolyte solution flow rate, and the size of the membranes, i.e., the residence time of the solution in the stack. In commercial operations, the residence time is large and the “acid-loop” solution 19 returning from the stack becomes very acidic, and can reach pH levels lower than 1. Similarly, the “base-loop solution” 21 can develop a very high pH such as 13.
An important parameter that dictates the effectiveness of bipolar electrolysis systems is the pH change of the solution as it passes through the ED stack. In certain configurations, it may be important to keep the pH change to a minimum, since high changes in pH may cause undesirable chemical changes to occur in the process solutions or may damage membranes or stack components. To prevent excessive drops (or rises) in pH, the present practice is to keep current density low, use high flowrates, or use short stacks to keep solution residence times low. These three factors result in lowered productivity per unit area of membrane, increased capital costs due to the necessity of using multiple short stacks instead of a few tall stacks, and increased operating costs due to excessive pumping energy.
Conversely, if the stack has greater height, operates at higher current density, and uses lower flow rates, the pH changes in the stack would cause the solution to become so acidic or basic as to damage the stack membranes, membrane spacers, and other components. More importantly, it may be impossible to produce the desired product due to large changes in solution pH.
U.S. Pat. Nos. 6,627,061, 6,331,236, 6,294,066, 6,224,731, and 6,221,225 awarded to Mani on Sep. 30, 2003, Dec. 18, 2001, Sep. 25, 2001, May 1, 2001, and Apr. 24, 2001, respectively, disclose an apparatus and method for electrodialysis of salts. Adjustment of pH is accomplished by adding base to an acid loop or adding acid to a base loop.
U.S. Pat. No. 6,461,491 awarded to Hryn, et al. on Oct. 8, 2002 discloses an apparatus and a method for electrodialysis. The method transfers dissolved salts or impurities in aqueous fluids from a diluate solution into a concentrate solution using ion-selective membranes. Acid or base is added to maintain pH level.
U.S. Pat. No. 5,268,079 awarded to Ochoa-Gomez, et al. on Dec. 7, 1993 discloses a method for the electrodialysis of salts of acids.
U.S. Pat. No. 5,198,086 awarded to Chlanda, et al. on Mar. 30, 1993 discloses a method for the electrodialysis of salts of weak acids and/or weak bases.
None of the aforementioned patents offer any narrow range of pH control or a means to internally maintain the pH at moderately acidic or basic levels during electrodialysis. Large stacks can yield enormous swings in pH which can cause process disruption and membrane damage. Without stringent pH control, they do not offer any means to prevent these potential problems. In addition, they do not offer any means for greater electrodialysis efficiency, e.g., higher current densities and greater throughput of product.
A need exists in the art for a method which provides for more cost-effective commercial-scale electrodialysis. The method should diminish or even eliminate net pH changes in the ED stacks to protect ED membranes. The method should, accordingly, allow for higher current densities, lower flow rates, and taller stacks to be more cost-effective. Finally, the method should be inexpensive.